1. Field of the Invention
The present invention relates to a multilayer capacitive element and a method for its production. More particularly, it relates to a multilayer ceramic capacitive element utilizing a lead-based complex perovskite for forming dielectric layers and comprising internal electrodes of copper or an alloy mainly comprising copper. Moreover, the invention relates to a method for manufacturing such multilayer capacitive elements.
2. Description of the Prior Art
Recently, multilayer ceramic capacitors have been widely popularized as there is an increasing demand for small-sized, large capacitance capacitors with severe requirements for size reductions of electronic devices as well as with increase in operating frequencies of electronic devices. In general, such a multilayer ceramic capacitor comprises a plurality of ceramic dielectric layers laminated one on another, and a plurality of internal electrodes separated by the ceramic dielectric layer and alternately terminated at opposite sides of the laminated ceramic dielectric layers.
In the production of such multilayer capacitors, green ceramic sheets are respectively applied with an internal electrode, laminated on one another, and then fired with internal electrodes at a sintering temperature of the dielectric ceramic material. Since the dielectric material conventionally used for multilayer capacitors are of a barium titanate system which has a sintering temperature of about 1300.degree. C., and since the internal electrodes are subjected to such a high sintering temperature, it is required to use, as a material for internal electrodes, a metal which does not react with dielectric and which has a high melting point and high resistance to oxidation. For that reason, noble metals such as gold, platinum, palladium and the like have widely been used for the internal electrodes. However, use of such a noble metal has led to increase in production cost of multilayer ceramic capacitors.
To solve that problem, it has recently proposed to produce multilayer ceramic capacitors utilizing a low sintering dielectric ceramic material together with an inexpensive internal electrode material. For example, Japanese patent application laid-open No. 62-210613 and No. 63-265412, and U.S. Pat. No. 4,752,858 disclose a multilayer ceramic capacitor comprising a plurality of dielectric layers of a lead-based complex perovskite, and internal electrode layers composed of copper or an alloy mainly containing copper, said lead-based complex perovskite containing a component (A) and a component (B), the component (A) being the one selected from the group consisting of Pb, Ca, Sr and Ba, but containing Pb at the least, the component (B) being at least two elements selected from the group consisting of Mg, Ni, Zn, Ti, Ta, Nb and W, the molar ratio of said component (A) to component (B), i.e., A/B, being not less than 1.00. In addition to the above application and patent, Japanese patent applications laid-open Nos. 62-115817, 62-128513 and 62-203321 each discloses a method for production of such multilayer ceramic capacitors.
The dielectric ceramic materials of the prior art can be fired at a relatively low sintering temperature of less than 1080.degree. C. in a reducing atmosphere, thus making it possible use copper or its alloy as a material for internal electrodes. It is, however, difficult to obtain dense ceramic dielectric which possess high specific resistance even at high temperatures, thus making it impossible to produce multilayer ceramic capacitors which possess high reliability even at high temperatures and high humidities.
To solve this problem, it has been proposed to incorporate a small amount of copper oxides (namely, 0.03 to 0.65 wt % in terms of Cu.sub.2 O) into the above lead-based complex perovskite dielectric material, for example, in U.S. Pat. No. 4,885,661.
In the production of such multilayer capacitors, however, it is required to fire the capacitors in a reducing atmosphere containing a reducing gas such as, for example, hydrogen, carbon monoxide, ammonia or the like with a low partial pressure of oxygen, as well as that disclosed in the above Japanese patent applications. For that reason, it is required to keep the partial pressure of oxygen in the firing atmosphere constant, thus making it considerably difficult to control the firing atmosphere. This may result in failure of mass production of multilayer ceramic capacitors with high reliability.
One idea to solve that problem is to fire green multilayer ceramic capacitors in a neutral atmosphere consisting essentially of nitrogen, but this has lead to problems. If the multilayer ceramic capacitors are fired in the nitrogen atmosphere, a part of copper in the electrode layers diffuses into the dielectric layers through the contacting surface between them during firing, resulting in a considerable difference in the content of copper oxides between the contacting and noncontacting areas of each dielectric layer with the electrode layer. Such a difference in the content of copper oxides causes production of cracks in the dielectric layers, and has led to decrease in the life characteristics of capacitors under the conditions of a high temperature and a high humidity.